An adjacent property between communication channels is high for the benefit of efficient using of a frequency spectrum in wireless communication nowadays. Accordingly, it is very important meaning to generating a clock and synthesizing a frequency.
A voltage controlled oscillator (VCO) is applied to the important components of phase lock loop (PLL) or frequency synthesizer. A voltage controlled oscillator (VCO) means the oscillator controlled frequency by voltage.
In the prior arts, LC-Tank oscillator is generally used for generating a clock and playing a voltage controlled oscillator. But the LC-Tank oscillator has low tuning range and high dependence on quality factor of phase noise in inductor. So, it is required to the separate process for abating phase noise. A ring oscillator, that it is high tuning range rather than the LC-Tank oscillator and may be integrated in CMOS process without a separate process, is generally used.
FIG. 1a shows a circuit diagram of a conventional ring oscillator. The conventional ring oscillator is configured to a cardinal number of delay cells connected parallel with one another.
Generally each delay cells configuring a ring oscillator is embodied to an inverter circuit. FIG. 1b shows a circuit diagram of each delay cells configured to an inverter, in the ring oscillator showed in FIG. 1a. 
By the circuit showed in FIG. 1b, first inverter 101 outputs power source voltage VDD or ground voltage GND according to the status of electrical potential. The output of first inverter 101 is inputted to second inverter 103 connected with first inverter 101 in series. Second inverter 103 outputs the voltage reversed to an input signal. The output of second inverter 103 is reversed by third inverter 105, and applied to the input stage of first inverter 101. If cardinal numbers of inverters are connected in series to have ring type connection, then, a clock signal having uniform frequency is acquired from an output of one of the inverters.
The conventional ring oscillator as mentioned above, it raises a serious problem of phase noise. So that the study for making up for the problem of the phase noise raised the ring oscillator, and differential delay cells are developed to the result of the study. Differential delay cells are superior to single-ended cells in noise characteristics, because their tail current that determines the delay is insensitive to the common-mode voltage variation. However, non-idealities of the circuit due to non-linearlities of load transistor and finite output impedance of tail current source prevent conventional delay cells from achieving good supply noise rejection. Moreover, considering the effect that 1/f noise of tail current is up-conversion, the phase noise performance of the differential delay cells may be worse than that of single-ended delay cells.
For solving above problem, delay cells not having the tail current source are developed in prior arts. These delay cells are solved above problem caused by the tail current, but it is raised the common-mode sensitivity problem of fully saturated VCO in case of using delay cells not having tail current, that is, it can not provide enough immunity to common-mode noise.